Existing x-ray film sensitometry typically uses an expensive combination of sensitometer and densitometer to evaluate the operation of film processing. The sensitometer (which costs from $250 to $650) produces three to twenty-one different exposures on a test film which is then processed in a facility's processor. The densitometer (which costs from $300 to $1000) evaluates the densities on the film to determine any changes from standard. Such changes can indicate a failure of an x-ray imaging/processing system which would possibly result in either excess exposure of a patient to radiation, to produce a useful radiograph, or would subject the patient to radiation exposure without yielding desired information on the radiograph. This process is currently used in larger x-ray facilities and does produce excellent results in the maintenance of daily quality control. The method does, however, require the expenditure of about $1000 to begin with, requires storage space, record keeping, and graphing of results, as well as considerable training of operating personnel.
Another approach is generically different from the above-described approach and results in simplification. The sensitometer is dispensed with and the x-ray generator is used to produce an exposure on a test film through a step wedge, usually a machined aluminum block producing 3-15 densities on the film. After processing, the test film may either be analyzed with a densitometer or by visual comparison with another similar 3-15 density step master film. Changes in the density of th steps of the test film from those of the master film indicate a possible problem. This approach affords significant improvement by providing monitoring of both the reproducibility of the performance of the x-ray apparatus and the x-ray imaging/processing system.
This last-named process is simple, but is difficult to apply directly to dental x-ray film (as done in standard sensitometry first-described above), since the films are so small as to preclude easy production or analysis of stepped images. Additionally, step wedge imaging requires an exposure different from that in typical use on patients and would not test system operation at patient use settings. Visual comparison of the step wedge images is difficult since the eye is called upon to make a match under distracting, confusing conditions, of numerous shades of image.
One comparison device, marketed by the Spectronics Corporation, would have exposures to a test film made under a standard condition and the test films compared to a standard control film. This assumes that there is an optimum condition that is suitable to all dentists, and the instructions on the Spectronics Corp. device assume that if the test film does not turn out light or dark enough, the exposure should be changed. This does not address the issue of controlling film processing, which is usually the major variable in system response.
A preliminary search of the prior art revealed the following prior U.S. patents of interest:
______________________________________ Eich, 1,953,471 Billing, 2,053,317 Capstaff, 2,326,007 Butler, 2,460,060 Bullock, 2,799,581 ______________________________________